1. Field of Invention
The present invention relates to mobile communication field, more specifically, to a method and a system for data retransmission in MBMS (Multimedia Broadcast/Multicast Service), wherein uplink feedback signalings are multiplexed/combined in one and the same uplink time-frequency resource block to thereby save resource consumption and to avoid uplink congestion.
2. Description of Prior Art
3GPP (Third Generation Partner Project) starts a LTE (Long Term Evolution) in 2005, which is expected to provide support for increasing demand by an operator and a user at a higher data throughput and a better network performance.
MBMS was introduced in 3GPP Rel6 and is a point to multipoint service for data transmission from a data source to multiple users. The MBMS can implement sharing of resources of networks (including a core network and an access network), and can provide a service to as many as possible multimedia users having the same demand at as less as possible resources. In a radio access network (RAN), by using a common transport channel and a common radio bearer, the MBMS can implement not only low rate multicasting and broadcasting of a textual message, but also high rate multicasting and broadcasting of a multimedia service (for example, a mobile TV).
Currently, study on EMBMS (Evolved MBMS) is in progress.
The present application is generally applied to a 3GPP LTE mobile communication system based on OFDMA (Orthogonal Frequency Division Multiple Access), especially to a design of EMBMS transmission in an evolved UMTS radio access network (E-UTRAN).
EMBMS is an evolution of MBMS in 3GPP LTE. By adopting a totally different physical layer transmission technique, i.e., OFDMA transmission mode, EMBMS has many new features as compared with MBMS in WCDMA (Wideband Code Division Multiple Access) Release6 protocols.
Transmission of MBMS service can be performed in a downlink in two ways, that is, a multi-cell transmission and a single-cell transmission. The present invention involves the single-cell transmission. In case of the single-cell transmission, i.e., cell-specific point to multipoint transmission, the multicast service is independent in each cell, and synchronous transmission is not needed. A MBMS traffic channel (MTCH) can be mapped to a DL shared data channel (DL-SCH).
If a UE (User Equipment) can not receive data synchronously from multiple asynchronous cells and combine them in evolved UTRA (E-UTRA), it can not achieve the same performance as MBMS soft combination in UTRA Rel16.
In E-UTRA, DL-SCH is mainly mapped by a dedicated traffic channel (DTCH). In order to improve a peak rate of a UE unicast traffic and a system throughput, DL-SCH is characterized by using HARQ (Hybrid Automatic Repeat Request), link adaptation based on dynamically varied modulation, coding and transmit power, dynamic and semi-static resource allocation, and CQI (channel quality indicator) reporting.
When the MTCH is mapped to the DL-SCH, the DL-SCH will be broadcasted in the entire cell. In order to improve data transmission rate and to reduce a block error ratio at edge of cells, the similar method and procedure to that of transmission of the unicast service, such as HARQ, can be applied to transmission of the multicast service.
In order to obtain precise and error free data transmission in data communications, combination of FEC (Forward Error Correction) coding with ARQ (Automatic Repeat Request) is usually applied to form an error control called as HARQ. The FEC can cause a receiver to obtain a correctly received data block or to judge that an erroneous data block is received. Thereinafter, the receiver sends a feedback message to the transmitter. If the data block is correct, an ACK (Acknowledged) message is sent. If the data block is erroneous, a NACK (Not Acknowledged) message is sent. The transmitter decides whether to transmit a next data block or to retransmit the erroneously received data block based on whether the received feedback message is ACK or NACK.
In prior art, HARQ procedure is always performed on a service with respect to one UE. That is, the UE receives unicast data, and transmits a NACK message for request of retransmission via the uplink channel when it is judged that the data block is erroneous. However, in case of multicast transmission in which the data is transmitted to multiple users, if many UEs receiving the data judge that the data block is erroneous, they will send NACK messages via the uplink channel simultaneously, causing congestion on the uplink channels. In connection with characteristics of design of the physical layer in E-UTRA, the applicant finds that uplink NACK signalings with respect to a service data packet can commonly use one and the same time-frequency resource.